This invention relates to a method for the manufacture of a reflection-proof film. In lenses for cameras and other optical instruments, the lens quality is improved by placing a multilayer reflection-proof film on the lens surface so as to decrease the amount of light reflected by the lens surface. The reflection-proof film may be of a single layer, depending on the particular purpose of use. Generally, however; it is formed of two or more optical layers, each of which is required to have a refractive index conforming to the refractive index of the substrate, such as glass, being overlaid by the film and at the same time to have a uniform composition throughout.
The methods for the formation of individual layers of a multi-layer reflection-proof film which have heretofore been disclosed to the art include those which resort to vacuum deposition.
Of the vacuum deposition methods suggested to date, particularly popular are the following methods:
A. A method whereby a single compound such as Na.sub.3 AlF.sub.6, MgF.sub.2, ZrO.sub.2, CeF.sub.3 or CeO.sub.2 is vacuum deposited on a substrate such as glass by means of electron beam or evaporation source utilizing a resistance heating means so as to form a vacuum deposited film having an optical film thickness of .lambda./4, .lambda./2, 3/4.lambda. (.lambda. = medium wavelength in the zone of wavelengths of the light whose reflection is required to be prevented), with the resultant film used as a component film for a multi-layer reflection-proof film.
B. A method whereby two substances each having a different refractive index are vacuum deposited to respectively suitable thicknesses by the method described in (a) above so as to form a symmetrical three-layer component film composed of layers having alternately different refractive indexes or to form a multi-layer film composed of a plurality of the said symmerical three-layer component films. In this case, the resultant film acquires properties of an equivalent film, namely a film possessed of substantially equivalent optical properties to those of a mono-layer film which has a given optical thickness such as .lambda./4, .lambda./2, or 3/4.lambda. and a given refractive index.
c. A method whereby two substances each having a different refractive index are mixed in a suitable proportion and the resultant mixture is vaporized by the agency of electron beam of evaporation source utilizing a resistance heating means so as to form a vacuum deposited film of a mixture having a given refractive index, with the resultant film used as a component film for a multi-layer reflection-proof film.
d. A method whereby two substances each having a different refractive index are vaporized at respectively suitable rates by two separate evaporating means so as to form a vacuum deposited film of a mixture having a given refractive index, with the resultant film used as a component film for a multi-layer reflection-proof film.
The conventional methods described above, however, suffer from the following drawbacks:
a. The vacuum deposited film produced from a single substance has a fixed refractive index as shown below. This method therefore, cannot produce a film having any other refractive index.
Na.sub.3 AlF.sub.6 -1.34; MgF.sub.2 -1.39; ZrO.sub.2 -1.95; CeF.sub.3 and Al.sub.2 O.sub.3 - 1.60; CeO.sub.2, ZnS, TiO and TiO.sub.2 - 2.3; ThO.sub.2 - 1.8; ThF.sub.4 and ThOF.sub.2 - 1.5
b. In order to obtain an equivalent film possessed of desired optical properties, it becomes necessary to effect rigid control of the thickness of individual film layers in the multi-layer film which is the equivalent film. Such rigid control requires an operation of very high technical level which cannot be obtained in commercial full-scale production.
c. The two substances vaporize at different temperatures. Consequently, the substance which has a lower vaporizing temperature is vacuum deposited before the other substance having a higheer vaporizing temperature has a chance to be vacuum deposited. Thus, it is not feasible to obtain a vacuum deposited film having a uniform composition throughout. The film consequently formed also has an optically heterogeneous structure. This method, therefore, fails to produce a practical reflection-proof film.
d. It is highly difficult to cause two substances to be vaporized from two separate vaporizing sources at such rates that there may be vacuum deposited in a fixed ratio of velocities. With this method, therefore, it is extremely difficult to produce a vacuum deposited film which is homogeneous in composition and has a given refractive index.